1. Field of the Invention
The present invention relates to a waveguide amplifier circuit which has a waveguide circuit and solid state amplifier elements therein which are subjected to simultaneous or parallel operation.
2. Description of the Prior Art
Conventionally, a high power amplifier for a very high frequency bandwidth comprises semiconductor amplifier elements in favor of long service life. However, since the semiconductor amplifier elements generate heat during operation, resulting in a temperature rise in the elements, they will not be allowed to consume power beyond a predetermined amount. Power outputs of the elements are thus limited. For this reason, a circuit is required which synthesizes very high frequency power outputs of these semiconductor amplifier elements.
From Michel Dydyk, "Efficient Power Combining" IEEE, MTT-S, 1970, International Microwave Sympodium Digest, pp 309-310, for example, the following power synthesis methods are known; a method in which reflex amplifier circuits are connected in series with each other such that their gain is controlled in accordance with output power of the semiconductor amplifier elements power; another method in which a waveguide amplifier circuit is connected in parallel with a hybrid circuit. However, when these methods are applied to waveguide circuits, the circuit arrangement become complicated. Meanwhile, when a waveguide circuit is arranged using a coaxial circuit such as a microwave strip, a circuit loss greatly increases.
In order to simplify the arrangement of the waveguide amplifier circuit, a reflex amplifier circuit can be conceivable wherein at least two semiconductor elements are arranged in a single waveguide.
FIG. 1a is a partially sectional plan view of an example of a conventional reflex amplifier circuit, while FIG. 1b is a side view thereof. Referring to FIGS. 1a and 1b, semiconductor elements 11 and 12 are arranged on a plane perpendicular to an electromagnetic wave propagation or travelling direction as shown by an arrow A, substantially making symmetry to a center line 30 of a waveguide 1. A portion 2 indicated by a curved arrow denotes a circulator. Reference numeral 21 denotes a coupling post. The amplifier circuit having the above arrangement is, however, liable to unstable operation at a frequency at which the electrical distance between the semiconductor elements 11 and 12 is half the wavelength of the electromagnetic wave propagating in the arrow B direction. It is therefore very difficult to arrange an unoscillatory power amplifier circuit which can produce a desired synthesized power stably. This is because electromagnetic coupling between two semiconductor elements is very close and the intensity of the electric field is zero at a middle point between the elements 11 and 12. Such nonuniform distribution of the electric field disables coupling to an external circuit. Furthermore, once this mode occurs, it cannot be easily attenuated, thereby resulting in abnormal oscillation.